replying to sms without coding the phone number in code;

i am creating so called project in which i want to reply to number whoever sends texts. without actually coding the phone number in code. project is based on automation so i dont know senders contact to hard code and it is not practically possible to do it. so if user x sends text it will perform the application and replys to x. and i dont know contact no. of x.need help..ass i am new please ignore the mistakes thank you....

Pleas post an example of the complete message that is received by SMS that you want to reply to